When we see records being broken and unprecedented events such as this, the onus is on those who deny any connection to climate change to prove their case. Global warming has fundamentally altered the background conditions that give rise to all weather. In the strictest sense, all weather is now connected to climate change. Kevin Trenberth

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New England, Tennessee, Oklahoma.... Who's next?

by Joe Romm, Climate Progress, June 14, 2010

I find it systematically tends to get underplayed and it often gets underplayed by my fellow scientists. Because one of the opening statements, which I’m sure you’ve probably heard is “Well you can’t attribute a single event to climate change.” But there is a systematic influence on all of these weather events now-a-days because of the fact that there is this extra water vapor lurking around in the atmosphere than there used to be say 30 years ago. It’s about a 4% extra amount, it invigorates the storms, it provides plenty of moisture for these storms and it’s unfortunate that the public is not associating these with the fact that this is one manifestation of climate change. And the prospects are that these kinds of things will only get bigger and worse in the future.

That’s Dr. Kevin Trenberth, head of the Climate Analysis Section at the National Center for Atmospheric Research, on the warming-deluge connection. I interviewed him a couple weeks ago about Tennessee’s 1000-year deluge aka Nashville’s ‘Katrina’.

Joseph Romm: I wanted to talk mostly about this Tennessee rain storm which didn’t get a lot of attention. And the potential link to global warming got virtually no attention at all. I sent you this link that the National Weather Service and NOAA put together of just how extreme an event it was.

Kevin Trenberth: Yes I just looked at it.

JR: This seems astounding to me. I’ve never seen anything like this. I’d just be interested in your thoughts on the unprecedented nature of this storm, and how you would characterize it in an age where scientists predicted as we warmed we’d get these kind of extreme deluges.

KT: That’s indeed true and that’s consistent with the expectations with regards to global warming. It’s also consistent with things that we’ve already seen … including happening in the US. So there was a study a few years ago now, that only went through 2002 but it was looking at the 20th century and at that point the average rainfall over the 48 contiguous states had gone up 7% but the heavy rainfall events had gone up 20%. And so the heavy rainfall events have been documented as increasing across the United States.

There was a more recent study that looked at somewhat different statistics and showed that the really heavy rainfall events — the top 1% and the top 0.3% — had gone up at even more alarming levels something like 27% as I recall over the last 30 or 40 years [actually 1967 to 2006]. And indeed most of those changes have occurred since about 1970.

Now the physical cause for this is very much related to the water vapor that flows into these storms. And these kinds of storms, well all storms for that matter, reach out on average — this is very much a gross average — about 4 times the radius or 16 times the area of the region that’s precipitating, the rain. And for these kinds of storms a lot of the moisture is coming out of the sub-tropical Atlantic and even the tropical Atlantic; some of it comes out of the Gulf of Mexico. And so the moisture actually travels about 2000 miles where it gets caught up in these storms and then it rains down. And the key thing is, that in the tropical and sub-tropical Atlantic the sea temperatures are at very high levels and in fact they’re the highest on record at the moment right in the eastern tropical Atlantic. It’s going to be interesting to see what that does for this hurricane season coming up.

For every one degree Fahrenheit increase in sea temperature, the water holding capacity for the atmosphere goes up by 4%. And since the 1970s on average there’s about a 4% increase in water vapor over the Atlantic Ocean and when that gets caught into a storm, it invigorates the storm so the storm itself changes, and that can easily double the influence of that water vapor and so you can get up to an 8% increase, straight from the amount of water vapor that’s sort of hanging around in the atmosphere. This is reasonably well established.

And so then, when you have the right conditions, and in the spring time, these conditions tend to occur, and the storm stalls a little bit (it ran up against a roadblock for a while), then this is one of the consequences.

So although a lot of aspects of this is the sort of thing that happens is weather and natural variability, one could easily argue that up to about a 10% enhancement of this is associated with global warming. And possibly even more in terms of some of the other things that are going on but which are much harder to pin down. So that’s the link.

It’s directly related to the fact that there’s more water vapor in the atmosphere. There’s warmer sea temperatures over the ocean that provide that moisture in the atmosphere. And then the right storm comes along, the right conditions, especially slow moving, and bingo.

JR: Stu Ostro, the senior meteorologist of the Weather Channel, he wrote a post on the storm that hit Georgia in the fall, and he talked about the atmospheric warming resulting in increase in the 1000 to 500 millibar thickness creating … basically an increase in 500 millibar heights.

KT: Right, It’s warmer in the lower half of the atmosphere, yes.

JR: This, what he called, “exceptionally strong ridges of high pressure sometimes accompanied by strong, persistent cut off flows…. So this is sort of another aspect of the climate change.

KT: Yes, The change in the storm tracks, or the change in the weather patterns is an aspect of it. That’s a little harder to pin down as directly relating to global warming. There might be some natural variability aspects to that, but you’ve only got to look around and so last year, if my memory serves me right, there was extensive flooding along the Mississippi River, and they were talking about 500 year storms, and yet in 1993 they had similar flooding along the Mississippi River, and again they were talking about 500 year storms. Here you are having these 500 year storms so to speak in slightly different areas but at the same time of year. This is a part of a pattern that exists.

We’ve seen other examples out in Seattle last year, and also of course the flooding in New England and the exceptionally heavy snow storms in Washington, DC, this year….

The same mechanism actually applies to the heavy snow, all you have to do is have the right weather conditions and for it to be cold enough and this precipitation just turns into snow. The very heavy snowfall amounts are actually related to the fact that the moisture that’s coming into that region is coming off of the tropical or sub tropical Atlantic where there’s abundant moisture and more moisture than there used to be: demonstrably more moisture than there used to be 30 years ago. So there’s a number of other examples you can point to as well.

JR: It seems to me the media hasn’t figured out a way to talk about this so they often just don’t talk about it at all.

KT: That’s correct.

JR: And as a result the public never learns the connection to climate change. I’m just wondering if you have any comments about that and what you would suggest is the right way to talk about it and the like.

KT:I find it systematically tends to get underplayed and it often gets underplayed by my fellow scientists. Because one of the opening statements, which I’m sure you’ve probably heard is “Well you can’t attribute a single event to climate change.” But there is a systematic influence on all of these weather events now-a-days because of the fact that there is this extra water vapor lurking around in the atmosphere than there used to be say 30 years ago. It’s about a 4% extra amount, it invigorates the storms, it provides plenty of moisture for these storms and it’s unfortunate that the public is not associating these with the fact that this is one manifestation of climate change. And the prospects are that these kinds of things will only get bigger and worse in the future.

KT: That’s right and so the weather events happened, the spring storms happen, you certainly expect these kinds of things at this time of year, but the odds are that when these happen getting flooding out of them is increasing substantially. And most of the time, even if there is this modest increase of say 5% or 5% to 10%, it’s still within the realm of natural variability frequently, but every so often you go outside of the realm of natural variability and that’s how you get these 100-, 500-, 1,000-year storm events that are occurring. And so this becomes the straw that breaks the camel’s back. Were just seeing more and more instances of that and the odds of that kind of thing happening are increasing.

JR: Yes, “Straw that breaks the camels back” is a phrase you used about Katrina as I recall.

KT: Again it’s the same sort of thing, where there’s an enhancement, there’s a global warming component. You can argue that it’s not the dominant component, especially on an individual storm like Katrina. But then in the 2005 season was just so exceptional, we had all these storms that went off in to the Greek alphabet, there was Rita and Wilma. Wilma, the strongest recorded storm on record. And Ophelia, that year, that churned around for 3 or 4 days right off of there coast of the Carolinas there. So it was not any individual storm there that you would really want to point to, but all these storms collectively are a clear indication that there was a warming component and at that points to the sea temperatures in the Gulf, and the Caribbean and in the tropical Atlantic which were by far the highest on record, and they were the fuel for that. As I say, this year in the Eastern Atlantic it’s even higher than that. But this year the Caribbean and the Gulf are somewhat cooler than average, and so how this hurricane season pans out is going to be an interesting one to see. But those are the factors that come into play.

JR: We’ve only warmed a little bit so far compared to what some of the models project. If we don’t reserve emissions trends soon, what kind of storms are we looking at in a few decades?

KT: The key number is this number in this case, which is pretty rock solid. For a 1 ° F increase in air temperature the water holding capacity goes up by 4%. Pretty close to 4%. And so if the sea temperatures go up by one degree, as they have, then the air temperatures probably go up a little bit more than that in fact. And so when you start talking about 3, 4, 5 degrees then you’re talking about 20% increases in the water vapor in the atmosphere.

The way in which we think this is going to happen is that the intervals between storms will be longer, but then when you do have the storms they are apt to be a doozie. When it rains it pours, so to speak. So you can really get deluges, and there are times when you have longer dry spells in between so there’s the risk of drought if you happen to miss these storms. And they are very much of a hit and miss nature. But then when you do get hit by them suddenly you’ve got a deluge.

And so this is a real challenge as to how you deal with that from a water management standpoint, from a drainage standpoint. In some sense, when this happens you’ve got all this water there, and water can actually be a valuable resource if you can put it into a reservoir, or a lake or something like that where you can use it in the future: it actually gets transitioned to a valuable resource. In the mean time, some of it just inundates areas and causes a major problem. And that relates to what kind of drainage systems you have, what kind of culverts you have. One of the ways in which city councils and the Corp of Engineers manage these things is to always build structures to deal with these things. But another key part of it is: can you put the water somewhere where you can use it in the future? Because these longer dry spells in between times are something else you’re going to have to contend with.

JR: Yeah and the issue is, of course, the rain when it comes down in the deluges , it overwhelms the capacity of the storm drainage and you get these runoff… We have so many combined sewage/storm water systems, we may have to rethink that…

KT: If they’re combined in that nature then you have water quality problems and you can get disease and cholera and things like that which can come out of that kind of situation. It’s the same thing even in open fields, that a lot more water gets transported across the surface of the field, and so all of the feces from animals can contaminate the water and cause various kinds of water contamination problems.